The present invention relates to a display in which pixel circuits are formed at intersections between signal lines and scan lines and therefore are disposed in a matrix, and particularly to a display employing organic electro-luminescence devices (organic EL devices) as its light-emitting devices. The invention also relates to a method of driving pixels in the display.
In recent years, increasing attention is being paid to organic EL displays as a flat panel display (FPD). Currently, liquid crystal displays (LCD) are predominantly used as FPDs. However, the liquid crystal displays are not self-luminous devices, and therefore requires additional components such as a backlight and polarizer. These additional components inevitably cause disadvantages such as an increase in the thickness of the display and insufficient luminance of the display.
In contrast, organic EL displays are self-luminous devices, and therefore require no additional components such as a backlight in principle. Accordingly, the organic EL displays are advantageous over the LCDs in terms of achieving a small thickness and high luminance of the display. In particular, active-matrix organic EL displays, in which switching devices are formed for each pixel, have advantages of achieving low current consumption due to hold-lighting of each pixel, and of allowing a large-size and high-definition screen comparatively easily. Therefore, the active-matrix organic EL displays have been developed in various manufactures, and are expected to enter the mainstream of future FPDs.
In recent years, personal imaging apparatuses typified by digital still cameras and digital camcorders have been developed. As a finder display device in these apparatuses, an LCOS (Liquid Crystal on Silicon), in which pixel circuits and drive circuits are formed on a crystalline silicon substrate, or a high- or low-temperature polycrystalline silicon LCD is used.
A finder employing a transmissive LCD requires a backlight, and one employing a reflective LCD requires a frontlight. Therefore, the finder employing an LCD inevitably involves a large module thickness, which leads to disadvantages for thickness reduction of the apparatus. In addition, in step with miniaturization of personal imaging apparatuses, the finder itself is miniaturized, which correspondingly reduce the size of pixels in the finder. Accordingly, in a transmissive LCD, it becomes difficult to ensure its aperture sufficiently, and the finder employing a transmissive LCD is getting close to its performance limit. As for a finder employing a reflective LCD, the LCOS is being brought into the mainstream. However, a lighting system is needed similarly, which does not contribute to thickness reduction of the apparatus.
In contrast, if an organic EL device is used as a viewfinder display device, the viewfinder display device can contribute to thickness reduction of the apparatus since the organic EL device is a self-luminous device and therefore requires no lighting system unlike LCDs. In addition, if an organic EL device having a top-emission structure is used, a sufficient aperture ratio for offering favorable performance can be ensured.
In recent years, viewfinders are tracking a trend toward a higher definition. Apparatus manufacturers have demanded for definition enhancement from QVGA (Quarter Video Graphics Array: 320×240 pixels) to VGA (Video Graphics Array: 640×480 pixels), and further to SVGA (Super Video Graphics Array: 800×600 pixels) and XGA (Extended Graphics Array: 1024×768 pixels).
In order to respond to these demands for a higher definition, use of a MOS process like the LCOS is required obviously. Furthermore, it is needed to decrease the number of devices in pixel drive circuits.
Typically pixel circuits for driving organic EL devices need to have a configuration for compensating variation in the threshold voltage and transconductance of transistors. Various techniques for the compensation configuration have been proposed. However, the drive circuit in most of these proposed techniques includes about five transistors. This number is large. In addition, a problem arises when transistors are formed by a MOS process. Specifically, the mobility of MOS transistors is in the range of about 300 to 600 cm2/V·s, and therefore the current supply ability of the transistors is too large to drive high-definition minute pixels.
As a circuit that is suitable for a MOS process and has a small number of devices, the circuit disclosed in PCT Patent Publication No. WO01/54107 is known. This pixel circuit is formed of two transistors and one capacitor.
Description will be made below about the conventional pixel circuit with reference to drawings. FIG. 13 illustrates the conventional pixel circuit. FIG. 14 shows the operation timing of the circuit of FIG. 13.
In the pixel circuit, all transistors are a P-channel transistor. The gate of a sampling transistor T11 is coupled to a scan line WS for controlling sampling of a video signal. The source thereof is coupled to a video signal line SIG, while the drain is coupled to one end of a capacitor Cs and the gate of a drive transistor T12.
The source of the drive transistor T12 is supplied with a supply voltage Vcc, and the drain thereof is coupled to the anode electrode of an organic EL device 4. The cathode of the organic EL device 4 is coupled to a line of a cathode supply voltage Vk.
The other end of the capacitor Cs is coupled to a line LVcs for supplying a voltage Vcs.
The operation of the pixel circuit will be described. At timing tm1 in FIG. 14, a scan pulse to the scan line WS is switched to a low potential, and thus the sampling transistor T11 is turned on. Thus, the potential at a node NA, which is equivalent to one end of the capacitor Cs, is set to the video signal potential. That is, a signal voltage Vs supplied through the video signal line SIG is written to the capacitor Cs.
At this time, the line LVcs for supplying the voltage Vcs to the capacitor Cs is fixed at a certain reference potential Vref (Vcs=Vref).
At timing tm2, the scan pulse to the scan line WS is turned to a high potential, which cuts off the sampling transistor T11. At the timing tm2, the voltage Vcs supplied from the line LVcs to the capacitor Cs is switched to a ramp signal voltage that repeatedly increases with time from the reference potential Vref to the maximum potential Vr. The cycle of the ramp signal is sufficiently shorter than one frame, and is typically set to one horizontal period.
After the timing tm2, in step with the increase of the voltage Vcs as a ramp signal, the potential at the node NA, i.e. the gate voltage of the drive transistor T12 increases from the signal voltage Vs toward the voltage Vs+Vr due to the capacitance coupling of the capacitor Cs. In the voltage increase period, the potential at the node NA reaches the cutoff voltage (the threshold voltage Vth) of the drive transistor T12 at certain timing. Thus, the drive transistor T12 is turned off, which stops supply of a current Iel to the organic EL device 4.
Until the cut-off of the drive transistor T12, that is, during the period when the drive transistor T12 conducts, the current Iel is supplied via the drive transistor T12 to the organic EL device 4, and therefore the organic EL device 4 emits light.
Such operation is implemented not only in the period from the timing tm2 to timing tm3, but also in the period from the timing tm3 to tm4, the period from the timing tm4 to tm5, and so forth. Specifically, after the video signal potential Vs is written in one horizontal period (e.g. tm1-tm2) within one frame, operation similar to that in the period tm2-tm3 is implemented based on a ramp signal in each horizontal period subsequent to the write period within the frame.
The drive transistor T12 operates in its linear region and thus is used as a switching device. Hence, during the period when the drive transistor T12 is in the on-state, the power supply Vcc is directly coupled to the anode of the organic EL device 4, and therefore the organic EL device 4 is driven under so-called constant-voltage drive.
Under the premise that the ramp signal waveform shows linear increases, the time period Ton during which the drive transistor T12 is in the on-state is expressed by Equation 1.Ton=(Vth/Vr)·Th+(Vcc−Vs)/Vr·Th  Equation 1
Note that in Equation 1, Vth denotes the threshold voltage of the drive transistor T12, Vr denotes the amplitude of the voltage Vcs, Vcc denotes the supply voltage, Vs denotes the video signal potential, and Th denotes the cycle of one horizontal period.
The time period Ton during which the drive transistor T12 is in the on-state is equivalent to the time period during which the organic EL device 4 emits light. Specifically, in one horizontal period (1H) for example, the organic EL device 4 emits light for the time period dependent upon the video signal voltage Vs supplied to the node NA. Gray-scale control is allowed by this light emission of the organic EL device 4 for the time period dependent upon the video signal voltage Vs.
Typically the threshold voltage Vth of a transistor varies over time.
Assuming that the threshold voltage Vth varies by ±ΔVth, Equation 2 is obtained from Equation 1.Ton=((Vth±ΔVth)/Vr)·Th+(Vcc−Vs)/Vr·Th  Equation 2
As Equation 2 shows, the ON time period Ton of the drive transistor T12 also varies.
However, the threshold voltage variation ΔVth of a MOS transistor is about ±10 mV. Therefore, if the ramp signal amplitude Vr is set to a sufficiently large value, e.g. to about 1 V, the threshold voltage variation ΔVth can be suppressed to about 1% of the amplitude Vr, which causes no problem in practice. That is, the ON time period Ton is not greatly affected by the threshold voltage variation ΔVth.
In addition, since gray-scale is controlled based on the ON time period Ton, if the ramp signal amplitude Vr is set to a large value, gray-scale offset and in-plane display roughness attributed to variation in characteristics of the drive transistor T12 among the pixels can be suppressed. Furthermore, since the cycle of the ramp signal equals to the cycle of one horizontal period, the ramp signal frequency is so high that no flicker arises.
However, in the conventional circuit like that shown in FIG. 13, a constant voltage is applied to the organic EL device 4 at the time of light emission thereof.
Typically an organic EL device driven with a constant current has a longer life than that of a device driven with a constant voltage. This respect will be described with reference to FIGS. 15A and 15B.
FIG. 15A shows the current-voltage characteristic (I-V curve) of an organic EL device. FIG. 15B shows the current-luminance characteristic (I-L curve) thereof.
Referring initially to the I-V curves of FIG. 15A, the characteristic of the device in the initial state is indicated by the full line, while the characteristic after deterioration thereof over time is indicated by the dashed line. In the initial characteristic, a voltage Vo offers a current Io. However, after deterioration over time, the same voltage Vo offers a current lower by ΔI than the current Io. That is, when the device is driven with a certain constant voltage Vo, a current flowing through the device decreases by ΔI after deterioration of the device over time.
Referring next to the I-L curves of FIG. 15B, the characteristic of the device in the initial state is indicated by the full line, while the characteristic after deterioration thereof over time is indicated by the dashed line. When the device is driven with a constant current, the luminance decrease associated with the deterioration over time is from the point <A> on the initial curve to the point <B>. In contrast, when the device is driven with a constant voltage, since the current decreases by ΔI as shown in FIG. 15A, the I-L deterioration further advances to the point <C>. That is, the degree of luminance deterioration is larger.
Therefore, constant-current drive is desirable in order to extend the life of an organic EL display. However, the conventional circuit shown in FIG. 13 cannot employ the constant-current drive.
As a circuit different from the circuit in FIG. 13, a pixel circuit that alleviates the influence of variation in transistor characteristics by using a ramp signal is disclosed in Japanese Patent Laid-open No. 2004-246320. The pixel circuit however is based on characteristics of low-temperature polycrystalline silicon, and therefore the number of devices in a basic circuit is large: seven transistors and one capacitor. Accordingly, the pixel circuit is unfavorable for high-definition pixels.
Under the above-described circumstances, there has been a need for a pixel drive circuit that achieves constant-current drive with a small number of devices and alleviates variation in transistor characteristics, to thereby allow a long-life, high-definition, and high-image-quality organic EL display.
In the pixel circuit shown in FIG. 13, during the period from the timing tm1 to tm2 of FIG. 14, for sampling a video signal, the supply voltage Vcc is applied to the organic EL device 4 almost independently of a gray-scale, and thus the current Ip flows through the organic EL device 4. That is, the organic EL device 4 enters a pseudo-emission state during the sampling of a video signal in the period from the timing tm1 to tm2.
In this case, the average current Iave in one frame is expressed by Equation 3.Iave={Ip+(Ton/Th)·(Nv−1)·Ip}/Nv  Equation 3
Note that in Equation 3, Ip denotes the peak current, Ton denotes the ON time period within one horizontal period, Th denotes the cycle of one horizontal period, Nv denotes the number of scan lines.
When black is displayed, Iave equals Ip/Nv since Ton equals 0. Therefore, floating black arises. When white is displayed, Iave equals Ip since Ton equals Th. As a result, the contrast ratio equals Nv. Therefore, the contrast ratio is defined by the number of scan lines, and a contrast ratio larger than Nv cannot be achieved in principle.
Thus, there has also been a need to achieve a pixel drive circuit that allows a long-life and high-definition organic EL display capable of displaying sharp images with a high contrast ratio.